Huntington's Disease (HD) is a neurodegenerative disorder that affects up to 1 in every 10,000 people. Although the mutation that causes HD, an expansion of CAG repeats encoding polyglutamine (polyQ) in the Huntingtin (Htt) protein, was discovered 20 years ago, there is still no effective treatment or cure for HD. My goal is to facilitate the understanding of HD pathogenesis by investigating the structure/function of Htt with the hope that this information will contribute to finding a cure for this disease. Htt is a large scaffolding protein (~350 kD) that is involved in multiple cellular functions. In all mammals, the polyQ stretch of htt is flanked by two domains that can modulate polyQ structure and toxicity; a proline-rich region (PRR) located at the C-terminus of the polyQ stretch, and a 17 amino acid N-terminal domain (N17) that forms an amphipathic helix capable of interacting with membranes and organelles [1-5]. The Htt N17 domain is highly conserved in vertebrates, and it is also the target for a number of post-translational modifications that can influence htt subcellular localization and the pathogenesis of mutant Htt [1, 4-9]. Recent in vitro experiments demonstrated that reduced htt expression or mutant htt expression affects the dynamic formation of cofilin-actin rods during ER/nuclear stress responses, and that the htt N17 domain is involved in this process [10, 11]. In addition, the N-terminal domain of Htt has been proposed to regulate Htt's function in autophagy [2, 12]. To explore further the contribution of the N17 domain to critical htt functions in vivo, I have generated a new mouse model expressing a version of htt lacking the N17 domain (Hdh?N17). Interestingly, mice homozygous for the N17 deletion are born at normal Mendelian frequency, suggesting that, like the PRR and the polyQ stretch [2, 3], the N17 domain is not required for htt's essential functions during embryonic development. The survival of Hdh?N17/?N17 mice provides me with an opportunity to characterize their behavior as they age (Aim 1), and my preliminary results indicate that the Hdh?N17/?N17 mice out-perform their control littermates in several behavioral tests. This is similar to observed phenotypes in mice expressing a version of htt lacking the normal polyQ stretch that leads to enhanced neuronal autophagy [8]. Thus, I will examine the possible role of the htt N17 domain in regulating htt's function in autophagy (Aim 2). In Aim 3, I will test the hypothesis that deletin of the N17 domain can affect normal htt's role in protecting against cellular stress. Finally, in Aim 4, I propose to assess the potential for the htt ?N17 deletion to modulate HD mouse model pathogenesis in trans, by characterizing Hdh140Q/?N17 mice. These analyses should provide much needed information about the contribution of the htt N17 to normal htt function in vivo, and its role in modulating HD pathogenesis.